Conventional oil leakage sensors are equipped with a resistor which comprises a drawn porous polytetrafluoroethylene resin with admixed fine carbon particles, and a covering material comprising drawn polytetrafluoroethylene which covers the resistor.
In oil leakage sensors, when oil which has passed through the continuous pores of the covering material reaches and permeates the resistor, the jumping of electrons between the fine carbon particles of the resistor is hindered so that the resistance of the resistor increases. As a result, the leakage of oil is detected.
Oil leakage sensors of this type allow oil to invade the sensor. However, since the covering material is water-repellent, the sensors prevent invasion of the sensor by water.
When the above conventional oil leakage sensors described above used in a high-humidity environment, the resistance increases in the absence of oil permeation just as it does in the case of oil leakage, so that false detection occurs.
When a conventional oil leakage sensor is used over a long period of time, the surfaces of the fine carbon particles are oxidized by oxygen and ozone in the air so that oxygen-containing functional groups such as carboxyl groups, carbonyl groups, and hydroxyl groups are produced. As a result of this oxidation, the active hydrogen on the surfaces of the fine carbon particles increases.
Meanwhile, although the covering material is water-repellent, it cannot prevent invasion by water droplets which are smaller than the diameters of the fine pores in said material. Accordingly, when such a sensor is used in a high-humidity environment, fine droplets of water reach the interior portions of the resistor. Molecules of the water undergo a hydration reaction with the functional groups formed on the surfaces of the carbon particles or bond with the active hydrogen on the surfaces of the carbon particles so that the surface conditions of the fine carbon particles change and the strength of the electrostatic field increases. As a result, conductivity is lessened and the resistance of the resistor increases.